Humanity is on course for a population greater than 11 billion by the end of this century, according to the latest analysis from the UN’s population division.

In a simple sense, population is the root cause of all sustainability issues. Clearly if there were no humans there would be no human impacts. Assuming you don’t wish to see the complete end of the human race – a desire that is shared by some deep green thinkers and [Bond super-villians](http://jamesbond.wikia.com/wiki/Hugo_Drax_(Michael_Lonsdale) – then the issue is whether there is an optimal number of humans on the planet.

Discussions on population growth often start with the work of Rev Thomas Robert Malthus whose An Essay on the Principle of Population published at the end of the 18th century is one of the seminal works of demography. Populations change in response to three driving factors: fertility – how many people are born; mortality – how many people die; and migration – how many people leave or enter the population.

Malthus observed that more births than deaths would lead to exponential growth which would always outpace any improvements in farming and increases in yields. Consequently, unchecked growth was doomed to end in famine and population collapse. Malthus was right about exponential growth, but he was famously wrong about his dire predictions for the consequences of such growth.

At a global level we can ignore migration (no interplanetary migration happening just yet) and so the tremendous rise in the total numbers of humans is a result of an imbalance between fertility and mortality rates.

Over longer timescales, the recent increases look practically vertiginous. We seem to be on a trajectory that would surely exceed whatever the carrying capacity of the Earth is. However, 11 billion could be the high water mark as the UN forecasts population to slowly decrease after the end of this century.

This brings us to Malthus’ first error: he wasn’t able to appreciate that the process of industrialisation and development that decreased mortality rates would, in time, decrease fertility rates too. Higher living standards associated with better education, in particular female education and empowerment, seem to lead to smaller family sizes – a demographic transition that has played out with some variations across most of the countries around the world.

This may explain how populations can overcome unsustainable growth, but it still seems remarkable that the Earth can provide for a 700% increase in the numbers of humans over the span of less than a few centuries. This was Malthus’s second error. He simply couldn’t conceive of the tremendous increases in yields that industrialisation produced.

How we fed seven billion

The “green revolution” that produced a four-fold increase in global food productivity since the middle of the 20th century relied on irrigation, pesticides and fertilisers.

You may describe yourself as an omnivore, vegetarian, or vegan – but in an sense we all eat fossilised carbon. This is because most fertiliser is produced through the Haber process which creates ammonia (a fertiliser) by reacting atmospheric nitrogen with hydrogen under high temperatures and pressures. All that heat requires serious amounts of energy, and the hydrogen is derived from natural gas, which currently means the Haber process uses lots of fossil fuels. If we include production, processing, packaging, transportation, marketing and consumption, then the food system consumes more than 30% of total energy use
while contributing 20% to global greenhouse gas emissions.

All this, just to grow some plants? Billingham is one of the UK’s largest fertiliser factories.Ben Brooksbank, CC BY-SA

Feeding the next four billion

If industrialised agriculture can now feed seven billion, then why can’t we figure out how to feed 11 billion by the end of this century? There may be many issues that need to be addressed, the argument runs, but famine isn’t one of them. However there are a number of potentially unpleasant problems with this prognosis.

First, some research suggests global food production is stagnating. The green revolution hasn’t run out of steam just yet but innovations such as GM crops, more efficient irrigation and subterranean farming aren’t going to have a big enough impact. The low-hanging fruits of yield improvements have already been gobbled up.

Second, the current high yields assume plentiful and cheap supplies of phosphorus, nitrogen and fossil fuels – mainly oil and gas. Mineral phosphorus isn’t going to run out anytime soon, nor will oil, but both are becoming increasingly harder to obtain. All things being equal this will make them more expensive. The chaos in the world food systems in 2007-8 gives some indication of the impact of higher food prices.

Third, soil is running out. Or rather it is running away. Intensive agriculture which plants crops on fields without respite leads to soil erosion. This can be offset by using more fertiliser, but there comes a point where the soil is so eroded that farming there becomes very limited, and it will take many years for such soils to recover.

Fourth, it is not even certain we will be able to maintain yields in a world that is facing potentially significant environmental change. We are on course towards 2℃ of warming by the end of this century. Just when we have the greatest numbers of people to feed, floods, storms, droughts and other extreme weather will cause significant disruption to food production. In order to avoid dangerous climate change, we must keep the majority of the Earth’s fossil fuel deposits in the ground – the same fossil fuels that our food production system has become effectively addicted to.

If humanity is to have a long-term future, we must address all these challenges at the same time as reducing our impacts on the planetary processes that ultimately provide not just the food we eat, but water we drink and air we breathe. This is a challenge far greater than those that so exercised Malthus 200 years ago.